(a) Field of the Invention:
This invention relates to a microcapsule-containing water-base coating formulation and a copying and/or recording material making use of the coating formulation. More specifically, this invention relates to a microcapsule-containing water-base coating formulation suitable for use in the production of a microcapsule-using copying and/or recording material with significantly-improved quality and productivity as well as the copying and/or recording material obtained by using the water-base coating formulation.
(b) Description of the Prior Art:
The history of microencapsulation goes back to the microencapsulation process making use of the gelatin wall complex coacervation technique, which was developed by The National Cash Resister Company as a result of an intensive research over many years. Use of such microencapsulation techniques has then been extensively attempted in a wide variety of application fields such as recording materials such as pressure-sensitive recording materials, pharmeceutical products, perfumes, temperature-indicating materials led by liquid crystals, foods, agricultural and horticultural chemicals, dyes, solvents, rust inhibitors, health-promoting foods, etc., leading to practical use of various products or tests therefor.
A number of proposals has been made, especially, on microcapsules of hydrophobic materials (oily materials and/or solids). Particularly, the coacervation process (phase separation process) making use of gelatin among the above proposals is practiced on a commercial scale mainly for carbonless copying paper.
However, microcapsules which are obtained by the complex coacervation process making use of gelatin and an anionic electrolyte of a high molecular weight are accompanied inter alia by the following problems:
(1) Since it is difficult to obtain microcapsules having a solid content higher than 20% due to the mechanism of the coacervation process, the microcapsules have low productivity per unit volume and require high transportation cost and when used as a coating material for carbonless copying paper and the like, a great deal of water has to be caused to evaporate for drying the coated materials, leading to still-standing serious problems on the efficiency of coating work and energy cost.
(2) Since the coacervation process employs a natural material for the formation of microcapsule walls, their quality and price are susceptible to greater fluctuations.
(3) Since the microcapsules tend to undergo putrefaction and coagulation subsequent to their preparation, they are not suited for long-term storage.
There is thus a strong demand for the improvement of such problems.
As improved techniques which are purportedly said to meet such a demand, some proposals have been made such as a process for the preparation of microcapsules from a urea-formaldehyde resin as a wall-forming material or a process for the preparation of a microcapsule slurry from a melamine-formaldehyde resin as a wall-forming material. Slurries of microcapsules of hydrophobic materials, which make use of these synthetic resins as wall-forming materials, have relatively high solid contents (30-50 wt. % or so) compared with microcapsule slurries obtained by the complex coacervation process and are thus excellent from the viewpoint of work efficiency and energy saving.
As microcapsules having high solid contents and superb quality, there have also been disclosed those obtained by using, as wall-forming materials, aminoaldehyde resins (urea-formaldehyde resins, melamine-formaldehyde resins, melamine-urea-formaldehyde resins, etc.) each of which features use of at least a multicomponent copolymer consisting as essential components of three or more acrylic monomers selected from (A) acrylic acid and/or methacrylic acid, (B) acrylonitrile and/or methacrylonitrile and (C) acrylamidoalkylsulfonic acid and/or sulfoalkyl acrylate, as an anionic water-soluble high polymer material. The above microencapsulation technique can provide microcapsule slurries of solid contents ranging from a low solid content to a super high solid content in excess of 60% while still maintaining their viscosities at low levels.
Microcapsules making use of the above-obtained various synthetic resins as wall-forming materials, especially, aminoaldehyde resins as wall-forming materials generally enjoy such advantages that they have higher solid contents, are excellent in terms of the denseness of their walls and are less susceptible to putrefaction or coagulation during their storage.
Microcapsules obtained in the above-described manner are generally of the pressure-rupturable type. Accordingly, when a liquid is used as a core material for the microcapsules, the microcapsules are susceptible to rupture due to pressure or frictional force during the preparation, finishing, selection and printing of base materials such as paper sheets coated with the microcapsules or their usual handling and applications so that they may develop smudge or their storability may be reduced. To cope with this problem, a water-base coating formulation composed of a microcapsule slurry and a stilt as a protective or buffer material for the microcapsules and a binder, which is generally soluble or dispersible in water, mixed in the slurry is prepared upon coating the microcapsules on a base material. Such a water-base coating formulation is then applied on a base material such as paper web usually by various coating methods (for example, by means of an air-knife coater and bar coater) or printing methods, followed by its drying.
As such a stilt, glass beads, finely-ground cellulose fibers (cellulose powder), ungelatinized starch particles (e.g., wheat starch, potato starch, pea flour starch) or the like is known. Generally, these stilts are inert particles somewhat greater (usually, 10-30 .mu.m) than microcapsule particles.
The stilt is mixed along with the other additive, namely, a binder, for example, a starch derivative (e.g., oxidized starch, esterified starch, etc.), a water-soluble high polymer material (e.g., polyvinyl alcohol, carboxymethylcellulose, hydroxyethylcellulose, acrylic acid base polymer, etc.) or a water-dispersible synthetic resin binder (e.g., various synthetic rubber latexes, vinyl acetate base emulsions, acrylic emulsions, etc.) in a microcapsule slurry to prepare a water-base coating formulation.
In order to prepare carbonless copying and/or recording materials for example, such a water-base coating formulation has such a weight composition that it contains 10-100 parts by weight of the stilt and 1-50 parts by weight of the binder per 100 parts by weight of the solid content of the microcapsules.
Taking by way of example pressure-sensitive copying paper which constitutes the greatest application field for such microcapsules, one of its constituent members, i.e., CB-sheets have heretofore been prepared by coating a water-base coating formulation such as that mentioned above, which contains as a principal component microcapsules enclosing as a core material a high boiling hydrophobic solvent with a triallyl-methanephthalide derivative or fluoran derivative dissolved therein, on a base material generally by means of an air knife or the like and then drying the thus-coated base material. On the hand, the other constituent member, i.e., CF-sheets are coated with a color-developing agent on the sides which oppose their matching CB-sheets when combined together. These coated surfaces are obtained by applying their respective high-density and high-viscosity coating formulations, the solid contents of which generally range from 50 to 70 wt. %, by means of a high-speed coating machine such as blade coater, roll coater or gravure coater. As has been already known in the art, water-base coating formulations are supposed to have relatively low viscosities of about 10-500 cps and relatively low concentrations (solid contents) of approximately 20-45% for their application on air-knife coaters. The upper limit of their coating speed is said to be 100-400 m/min or so.
On the other hand, water-base coating formulations containing a color-developing agent generally have solid contents of 50-65% and viscosities of 200-5,000 cps and their coating speed is as high as 400-1,000 m/min. Under the circumstances, there is a significant difference in productivity between the coating step for a coating formulation of microcapsules and that for its corresponding coating formulation of a color-developing agent. It is hence a common desire for the present field of art to improve the productivity of the coating steps of microcapsule-containing coating formulations.
The following two reasons may be mentioned as major causes which have prevented improvements to the productivity through high-concentration and high-speed coating of water-base coating formulations of microcapsules:
(1) It was difficult by conventional microencapsulation techniques to obtain a microcapsule slurry of a such high solid content that would permit the preparation of a high-concentration coating formulation.
(2) A stilt employed as a buffer material against pressure on surfaces coated with such a coating formulation of microcapsules was scraped off by a blade while the coating formulation is applied by the blade or gravure coating technique which is a typical example of high-speed coating techniques. As a result, the amount of the stilt still remaining on the coated surface was reduced significantly. This rendered the coating layer of the microcapsules excessively sensitive to pressure, resulting in the tendency of microcapsule rupture and smudge development by pressure or frictional force during preparation, finishing and/or printing steps or during usual handling.
Pertaining to the above cause (1), i.e., the preparation of a microcapsule slurry of a high solid content, techniques have been established for the preparation of microcapsule slurries having such high solid contents, that have not been achieved by any conventional techniques, as a reflection of recent advancement in the microencapsulation techniques which make use of synthetic resins as wall-forming materials. Especially, according to the microcapsule preparation process which makes use of an aminoaldehyde resin as a wall-forming material and was proposed by the present inventors, a microcapsule slurry having a super high solid concentration in excess of 60% may be obtained with a low viscosity.
Although it has been succeeded to obtain a microcapsule slurry of such a high solid content, a stilt is indispensable for copying and/or recording materials making use of such microcapsules as mentioned in the above cause (2). Under the circumstances, coating of such a stilt along with microcapsules on a surface of a base material cannot be effected unless water is added to the coating formulation to lower its viscosity and concentration and the thus-adjusted coating formulation is applied at a relative low speed by means of an air-knife or bar coater.
In other words, it has become feasible to coat a microcapsule-containing water-base coating formulation at a relatively high concentration compared with conventional coating formulations owing to the success in the preparation of the starting microcapsule slurry with a higher solid content. However, the productivity of CB-sheets is still far lower compared with CF-sheets. Moreover, a great deal of water must be dried off from the base material subsequent to its coating and substantial energy is hence required for its drying.
In the field of copying and/or recording materials making use of such microcapsules, specifically, in the field of carbonless recording materials, self-contained carbonless copying paper is prepared by applying microcapsules, which enclose a colorless dyestuff precursor, and color-developing agent (usually, an oil-soluble acidic material of organic nature) on the same surface of a base material. The microcapsules are ruptured by pressure to induce a color-producing reaction between the dyestuff precursor and color-developing agent, thereby obtaining recorded marks.
Self-contained carbonless recording sheets which are presently in use are primarily self-contained carbonless recording sheets of the double-layered coating type, one of which is obtained by coating a layer of microcapsules, in which a dyestuff precursor is enclosed, and a color-developing layer over the former layer on one side of a base material. Reflecting recent advancement in the microencapsulation technology, self-contained carbonless recording sheets of the single-layered type have also been proposed and have partly been put in practical use.
In the case of a self-contained carbonless recording sheet of the single-layered coating type, a dyestuff precursor (e.g., a phthalide type compound, fluoran type compound, or the like) and a color-developing agent (e.g., an oil-soluble phenol resin, salicylic acid derivative, or the like) are individually microencapsulated. The resulting microcapsules are mixed together into a homogeneous coating formulation, which is then coated as a single layer.
Turning to a self-contained carbonless recording sheet of the double-layered coating type, a layer of microcapsules enclosing a dyestuff precursor and a layer of a color-developing agent are coated one over the other in two layers as mentioned above. Such self-contained carbonless recording sheets of the double-layered coating type are however still accompanied by problems in both production cost and performance for the following reasons:
(1) It is only possible to obtain coating formulations of low concentrations since microcapsules of dyestuff precursors are prepared by using, as their microencapsulation technique, the complex coacervation process making use of gelatin as a wall-forming material.
(2) They require two coating layers, leading to very poor productivity.
(3) Since the color-developing layer and its associated layer of microcapsules of a dyestuff precursor are provided separately, it is impossible to achieve excellent color-producing speed and color density.
On the other hand, conventionally-known self-contained carbonless sheets of the single-layered coating type enjoy significantly-improved productivity such as completion of coating in a single step and improved yields. In addition, they have another advantage that high color densities can be easily obtained by color-producing processing because the dyestuff precursors and their corresponding color-developing agents are located close to each other in their entirety They are however accompanied by such problems that microencapsulation of the color-problems developing agents is also required to avoid the problem of accidental color development before subjecting them to color-producing processing, to say nothing of the microencapsulation of the dyestuff precursors, resulting in need for an extra expense for the microencapsulation of the color-developing agents, and undesirable color development (smudge) occurs more easily by friction, paper folding or the like due to the structures of their coated surfaces compared with the self-contained carbonless sheets of the double-layered structure. These problems have not yet been completely solved.
Furthermore, self-contained carbonless recording sheets involve a fundamental problem that their marks have inferior solvent resistance and are readily faded out upon contact with a polar solvent, for example, a plasticizer such as an ester of phthalic acid and may hence be rendered illegible subsequent to their recording.